The demodulation of modulated light signals at the pixel level requires in current approaches, the switching of a photo-generated charge current. Basically, both electron as well as hole currents are possible. The common methods use the photo-generated electron currents due to their higher mobility in the semiconductor material. Some pixel architectures do the necessary signal processing based on the photo-charge currents whereas other architectures work in the charge domain.
Common to all pixels is the necessary transfer of photo-charges through the photo-sensitive detection region to a subsequent storage area or to a subsequent processing unit. In the case of charge-domain based pixel architectures, the photo-charge is generally transferred to a storage node. In order to demodulate an optical signal, the pixel has to have at least two integration nodes that accumulate the photo-generated charges during certain time intervals.
Different pixel concepts have been realized in the last few decades. U.S. Pat. No. 6,825,455 to Schwarte introduced a demodulation pixel, which transfers the photo-generated charge below a certain number of adjacent poly-silicon gates to discrete accumulation capacitances. U.S. Pat. No. 5,856,667 to Spirig et al. disclosed a CCD lock-in concept that allows the in-pixel sampling of the impinging light signal with theoretically an arbitrary number of samples. Another similar pixel concept has been demonstrated by T. Ushinaga et al., “A QVGA size CMOS time-of-flight range image sensor with background light charge draining structure”, Three-dimensional image capture and applications VII, Proceedings of SPIE, Vol. 6056, pp. 34-41, 2006, where a thick field-oxide layer is used to smear the potential distribution below the demodulation gates.
A common problem of the afore-mentioned pixel approaches is the slowness of the photo-charge transport through the semiconductor material. This decreases significantly the accuracy of the in-pixel demodulation process. In all pixel structures, the limiting transport speed is the step-wise potential distribution in the semiconductor substrate that is used to transport the charges through the semiconductor in lateral direction. In those configurations, thermal diffusion dominates the transport speed instead of the fast movement by lateral electric drift fields.
New concepts of pixels have been explored in the last years accelerating the in-pixel transport of the charges by exploiting lateral electric drift fields. Seitz disclosed in U.S. Pat. No. 7,498,621 a first drift field demodulation device that is based on a very high-resistive poly-silicon gate electrode. It even allows the design of pixels having an arbitrary number of samples. Van Nieuwenhove et al., in “Novel Standard CMOS Detector using Majority Current for guiding Photo-Generated Electrons towards Detecting Junctions”, Proceedings Symposium IEEE/LEOS Benelux Chapter, 2005, introduced another drift field pixel, where a drift field in the substrate is generated by the current of majority carriers. To perform demodulation of photo-generated minority carriers, the majority current is dynamically controlled by the modulation signal.
The aforementioned drift field pixel concepts, however, have two drawbacks: First, the demodulation requires the switch of large capacitances since the whole sensitive area needs to be controlled. Second, an electronic current is used to generate the drift fields, which leads to a significant in-pixel power consumption.
An alternative pixel concept is described as the static drift field pixel and is disclosed in US 2008/0239466 A1 by Buettgen, which overcomes these two problems. In contrast to the architectures mentioned above, it clearly separates the detection and the demodulation regions within the pixel. It shows lower power consumption and, at the same time, it supports fast in-pixel lateral charge transport and demodulation.
One major application of demodulation pixels is found in real-time 3-D imaging. By demodulating the optical signal and applying the discrete Fourier analysis on the samples, parameters such as amplitude and phase can be extracted for the frequencies of interest. If the optical signal is sinusoidally modulated, the extraction based on at least three, but most commonly implemented four discrete samples will lead to the offset, amplitude and phase information. The phase value corresponds proportionally to the sought distance value. Such a harmonic modulation scheme is often used in real-time 3-D imaging systems incorporating the demodulation pixels.
Depending on the above-mentioned pixel architectures that is used to build the high-speed charge transfer, there are limitations in building a pixel enabling the sampling of four times and to store those four values. State-of-the-art pixel architectures, however, can only sample the two opposition phases. Hence, only two samples can be stored by one acquisition. At least a second acquisition is required to be able to reconstruct phase, amplitude and offset of the modulation light.